This invention relates to dock leveler construction and in particular to an improved support system in the form of trapezoidal shaped beams which support the deck plate of the leveler.
Dock levelers are used in the material handling industry at loading docks to provide a bridge between a dock and the bed of a vehicle which is loaded and/or unloaded. These devices are either pit or box mounted and may store either vertically or horizontally. Actuation of the dock leveler is by use of hydraulics or mechanical spring systems.
During loading and unloading operations there is a difference in the level between the dock leveler deck and the bed of a truck. Thus, some technique must be used to compensate for the difference in level. Systems in use employ either a flexible deck with rear hinges or a rigid deck with floating rear hinges. In the case of a flexible deck, beams are used which must have a low torsion rigidity. Examples are the C-channel, the I-beam, or in some cases special beam arrangements such as T or Lambda beams. The T beam construction is illustrated in the context of the leveler in U.S. Pat. No. 3,587,126. The so called lambda beam is illustrated in a leveler in U.S. Pat. No. 4,086,338.
A leveler employing a rigid deck construction is generally strengthened by having the deck plate welded to a box beam to provide a complete composite beam structure. In practice, the box beam is formed using a channel which forms a complete box when the two free legs are welded to the deck plate. Structural analysis has demonstrated that a box beam provides the most efficient structure for a dock leveler deck assembly.
The box beam when compared to all other beam sections with the exception of a L or T beam has a higher proportion of material at the maximum distance below the deck plate. This provides a stiffer composite beam structure for a given weight of steel.
Each box beam flange provides two support ribs for the deck as compared to a single support area for a Channel, I beam, L or T beam. This in turn reduces the span of deck plate between the beams and provides the deck plate with better strength and stiffness to resist highly concentrated loads such as those exerted by a fork lift truck traversing the deck plate. The bending stress caused by the load on the plate between the beams is proportional to the square of the span between adjacent beams. The deflection is proportional to the cube of the span. Also, the box beam is easier to assemble because it is inherently stable when set in place during assembly. A Channel section is marginally stable but an L or T section is unstable and must be held in place or it may fall over.
U.S. Pat. No. 4,928,340 defines an improvement in box beam technology by providing split box beams to insure torsional flexibility in dock leveler systems that employ rigid hinge pin construction. This technique is effective to solve the problems with rigid pin systems. It however provided no more support than a conventional box beam to the deck.